As is well known in the field of polymer chemistry, diblock copolymers (DBCPs) comprise molecules having two polymer blocks joined together by a covalent bond. DBCPs can come in many forms, such as those disclosed in M. Park et al., “Block Copolymer Lithography: Periodic Arrays of ˜1011 Holes in 1 Square Centimeter,” Science, Vol. 276, pg. 1401, May 30, 1997) (the “Park Reference”), which is incorporated herein by reference. As disclosed in the Park Reference, DBCPs can include molecules with blocks of polystyrene and polybutadiene (PS-PB), or polystyrene and polyisoprene (PS-PI). Other references disclose DBCP molecules with blocks of polystyrene and polymethylmethacrylate (PS-PMMA), yielding a conjoined molecule having a molecular weight of 70 kg/mol and a 7-to-3 mass ratio of PS to PMMA.
Because the block components are not miscible, the DBCP molecules will tend to align to bring like blocks (or ends of the molecule) together when energy is added to the film. Thus, when the DBCP solution is placed on a support structure (such as a silicon substrate or other film placed thereon), like ends of the molecules will draw together to form small cylindrical domains in the DBCP film. Such domains may be cylindrical or spherical in nature depending on the relative polymer chain lengths and on the surface binding energies. More specifically, the molecules will aligned when the glass transition temperature is exceed for the DBCP film in question (e.g., approximately 125° C. for a PS-PB DBCP).
Such cylindrical domains in a thin film will naturally tend to become as closely packed as possible, and hence generally take on a hexagonal or “honeycomb” appearance in the DBCP film. This is shown in FIG. 1, which shows from a top view cylindrical domains of PB 16a in a matrix of PS 16b. The domains 16a typically have a diameter of approximately 10-20 nanometers (“d”). Moreover, typical domains are spaced from one another at their centers by 40 nanometers (“x”), what is referred to as the domain spacing. The sizes and spacing of the diameters of the domains (“d”) as well as the domain spacing (“x”) depend on the relative sizes (e.g., chain lengths) the polymers blocks used in the DBCP, and will vary between different DBCP formulations. The exact dimensions “d” and “x” for a given DBCP film are usually well known and can be well tailored for a given application.
Because each of the block components are sensitive to chemicals that the other is not, one of the block components can be selectively removed in the DBCP film, leaving either the cylindrical domains (e.g., of PB) or holes where the cylindrical domains used to appear (e.g., of PS), which provides creative masking solutions for underlying structures.
Exemplary processes for removing one of the block components and for etching an underlying support structure are disclosed in the Park Reference, and are briefly illustrated in FIG. 2. FIG. 2A shows initially that a DBCP layer 16 has been placed on a layer 11 to be etched upon substrate 10. Should it be desired to leave domains 16a (i.e., to remove the PS) as an etch mask, the DBCP layer 16 can be treated with osmium, and more specifically an OsO4 vapor (referred to herein as “Process I”). This treatment “stains” the PB domains 16a by adding osmium across the carbon-carbon double bonds in the PB backbone, making the domains more resilient to the plasma etchant used to etch the underlying layer 11, as shown in FIG. 2B. After such plasma etching, the remaining domains 16a are removed (FIG. 2C), leaving “dots” of layer 11. Should it be desired to remove domains 16a (i.e., to remove the PB) as an etch mask, the DBCP layer 16 can be treated with ozone (referred to herein as “Process II”). Ozone predominantly attacks the carbon-carbon double bonds in the PB domains 16a, cutting the bonds and producing PB fragments that can be removed with water. This results in voids in the PS matrix 16b, thus largely exposing the underlying layer 11 in the locations where the PB domains 16a used to be present (FIG. 2D). The layer 11 can then be plasma etched using the PS matrix 16b as a mask (FIG. 2E). After such plasma etching, the remaining portions of the PS matrix 16b are removed (FIG. 2F), leaving “holes” in layer 11.
While DBCPs can be used in a variety of masking applications to fabricate different types of structures for differing purposes, they find particular utility in the manufacture of memory cells. For example, FIG. 3 demonstrates using a DBCP layer to form flash EPROM memory cells using Process I discussed above. In this example, as shown in FIG. 3A, a silicon crystalline substrate 10 is layered with a tunnel dielectric 12 (e.g., silicon dioxide or nitride) and a polysilicon layer 14. A DBCP layer 16 is deposited on the polysilicon layer 14 and processed to form domains 16a therein. The DBCP layer 16 is treated (with osmium) and the structure is etched (FIG. 3B), thus etching the polysilicon to leave polysilicon dots 14a. Once the domains 16a are removed (FIG. 3C), a control gate dielectric 17 (e.g., silicon dioxide or nitride) is formed over the resulting structure (FIG. 3D). Then a second layer of polysilicon 18 is deposited and etched using traditional patterning and etching techniques (FIG. 3E), and which might have a width (w) as small as 100 nanometers or so. Thus, a flash memory cell is formed, in which charge is selectively storable on the polysilicon dots 14a, which in tandem act like a single floating gate in a traditional flash memory cell and which are controllable by control gate layer 18.
A modification to the process flow for forming a flash memory cell using a DBCP layer and Process II is illustrated in FIG. 4. As shown in FIG. 4A, a thick dielectric layer 12 (which eventually will become in part the tunnel dielectric 12) is formed on the silicon crystalline substrate 10. The DBCP layer 16 is formed on the dielectric layer 12, and again separated into its constituent components 16a and 16b. In this modified process, the PB domains 16a are treated (with ozone) and removed (FIG. 4B). Remaining portions 16b act as the mask for the underlying dielectric layer 12, which is plasma etched to form cylindrical holes 12a whose bottoms constitutes tunnel oxide 12 (FIG. 4C). Thereafter, polysilicon is deposited on the resulting structure and etched back to at least partially fill the cylindrical holes 12a to form polysilicon dots 14a (FIG. 4D). Then the control gate dielectric 17 is deposited (FIG. 4E) and the control gate layer 18 of polysilicon is patterned and etched (FIG. 4F).
However, while useful as masking layers, DBCP films as used in the prior art are not ideal, especially when applied to the formation of memory cells such as those illustrated above. First, the number of cylindrical domains formed in the patterned layer (e.g., polysilicon domains 14a) will not always appear in a predictable relationship with respect to other structures traditionally patterned elsewhere on the device. For example, and referring to FIG. 5A, the relationship between the polysilicon dots 14a and the overlying control gate 18 (in dotted lines) are shown. As can be seen, the control gate 18 does not entirely cover a discrete number of polysilicon dots 14a: some dots 14a (e.g., 19) are only partially covered by the control gate 18 at its edge. Thus, as alignment varies from control gate to control gate (or from device to device), the numbers of polysilicon dots 14a covered (and thus affected) by the control gate 18, and/or the extent of that coverage, will change. Even if the control gate could be consistently aligned and patterned with extreme precision (+/−1 nanometer or so), the position of the polysilicon dots 14a will change from control gate to control gate because they are defined by the DBCP film, which in turns grows its domains at random starting locations on the device. The effect is that each control gate 18 (or memory cell) will have slightly different electrical properties due to the different numbers and orientations of the dots 14a. Such variability in the finished device is of course not optimal.
Another problem (which exacerbates the first) is that the domains in the DBCP layer will not form on the device with perfect uniformity. In this regard, the DBCP layer is akin to polycrystals. Thus, the DBCP layer will establish local regions of perfect order, or “grains” 20, but on the whole will have inconsistency in its ordering, as is shown in FIG. 5B. (Typically, the grain size of such grains 20 may be on the order of ten domains or so, but this is variable and depends on the temperatures and times used during domain formation). Thus, it is seen that the DBCP layer in FIG. 5B (as reflected in the eventual locations of the polysilicon dots 14a) had at least three fairly distinct grains 20a-20c defining grain boundaries 21 in between. More generally however (and possibly as a result of grain formation), certain domains or areas of domains 22 are disordered when compared to the predominant local ordering that is present. The salient point is that the domains in the DBCP layer, and hence resulting structures etched thereby such as the polysilicon dots 14a, will vary in their order. Again, this injects variance into devices formed using such structures (e.g., memory cells), as each control gate 18 may have slightly different numbers or arrangements of polysilicon dots 14a that it can influence (or that it is influenced by).
Richard D. Peters et al., “Combining Advanced Lithographic Techniques and Self-Assembly of Thin Films of Diblock Copolymers to Produce Templates for Nanofabrication,” J. Vac. Sci. Tech, B 18(6), pp. 3530-34 (November/December 2000), which is hereby incorporated by reference, suggests a method to more accurately order the domains as they form in the matrix. In Peters, a substrate was treated with an “imaging layer,” such as an alkylsiloxane layer. The imaging layer is patterned using extreme ultra-violet (EUV) or X-ray radiation to form chemically-altered stripes whose period roughly match that of the domain spacing, x. Because these chemically-altered stripes selectively wet to the domains, the domains will tend to form above them, leaving the matrix portions of the copolymer in the unexposed portions of the imaging layer between the chemically-altered stripes (anti-stripes). The diblock copolymer is then formed over the imaging layer and heated to promote domain formation, such that the domains form in straight lines over the chemically-altered stripes, and the matrix portion forms in straight lines over the anti-stripes. Then, either the domains or the matrix are removed, and used as a template.
However, Peters' approach is not suitable for some applications. First, it requires the use of an imaging layer, which introduces potential contamination and complexity to the process. Second, while promoting domain order, such domain ordering is formed along straight lines. This is useful for patterning lines in the underlying circuit layer, but not dots or holes, and thus is not useful in all applications.